op {
  graph_op_name: "Igamma"
  summary: "Compute the lower regularized incomplete Gamma function `P(a, x)`."
  description: <<END
The lower regularized incomplete Gamma function is defined as:


\\(P(a, x) = gamma(a, x) / Gamma(a) = 1 - Q(a, x)\\)

where

\\(gamma(a, x) = \\int_{0}^{x} t^{a-1} exp(-t) dt\\)

is the lower incomplete Gamma function.

Note, above `Q(a, x)` (`Igammac`) is the upper regularized complete
Gamma function.
END
}
